DESCRIPTION(From applicant's abstract): This application is a response to the request for application (RFP) 'Earth-based Research Relevant to the Space Environment' (PA-00-088). It concerns the enzyme serotonin N-acetyltransferase, responsible for the daily rhythmic production of melatonin. Melatonin is an animal and human hormone thought to be involved in the regulation of circadian rhythm, reproductive physiology, and mood. Peak melatonin levels appear to fall off with increasing age. Produced in the pineal gland, melatonin levels vary up to 100-fold over a 24 h day, primarily in response to external light, posture, and changes in magnetic fields, and this response is mediated by serotonin N-acetyltransferase. Despite the importance of serotonin N-acetyltransferase, there is currently a poor understanding of its enzyme mechanism. No specific inhibitors of serotonin N-acetyltransferase that are active in vivo have been reported. The main goals of this application are to develop specific and potent inhibitors of serotonin N-acetyltransferase. Employing substrate analogs, mutagenesis, X-ray crystallography, and kinetic studies, insights into the transition state of the catalytic mechanism will be obtained which will be applied to the design of selective enzyme inhibitors. The design and synthesis of inhibitors will be in part based on mechanistic studies as well as a systematic analysis of functional group substitutions. Potent inhibitors will be analyzed in vitro to define their specificity and mechanism of action and promising compounds will be tested in vivo cell culture assays. Specific inhibitors of serotonin N-acetyltransferase will help clarify the role of melatonin in circadian rhythm biology in humans on the ground and during space travel and will be potentially useful as therapeutic agents for a variety of clinical conditions that can affect civilians as well as astronauts, including sleep and psychiatric disorders. Furthermore, such inhibitors could contribute to an understanding of the physiologic and pathophysiologic effects of melatonin changes in the elderly by mimicking in animal models the alterations in melatonin production that occur with aging.